How Chromosomes Determine Sex How Chromosomes Determine Sex Basic Genetics How Chromosomes Determine Sex A karyotype is the complete set of chromosomes of an organism. Humans have 46 chromosomes in total: 23 inherited from the mother (blue) and 23 from the father (pink). The sex chromosomes (labeled XY and XX) determine the individual's sex. Credit: PASIEKA/SPL/Getty Images By Regina Bailey How Chromosomes Determine Sex Chromosomes are long, segments of genes that carry heredity information. They are composed of DNA and proteins and are located within the nucleus of our cells . Chromosomes determine everything from hair color and eye color to sex. Whether you are a male or female depends on the presence or absence of certain chromosomes. Human cells contain 23 pairs of chromosomes for a total of 46. There are 22 pairs of autosomes (non-sex chromosomes) and one pair of sex chromosomes. The sex chromosomes are the X chromosome and the Y chromosome. Sex Chromosomes: In human sexual reproduction, two distinct gametes fuse to form a zygote. Gametes are reproductive cells produced by a type of cell division called meiosis . Gametes are also called sex cells .

They contain only one set of chromosomes and are said to be haploid . The male gamete, called the spermatozoan, is relatively motile and usually has a flagellum . The female gamete, called the ovum, is nonmotile and relatively large in comparison to the male gamete. continue reading below our video Test Your General Science Knowledge When the haploid male and female gametes unite in a process called fertilization , they form what is called a zygote. The zygote is diploid , meaning that it contains two sets of chromosomes . Sex Chromosomes X-Y: The male gametes or sperm cells in humans and other mammals are heterogametic and contain one of two types of sex chromosomes . They are either X or Y. The female gametes or eggs however, contain only the X sex chromosome and are homogametic . The sperm cell determines the sex of an individual in this case. If a sperm cell containing an X chromosome fertilizes an egg, the resulting zygote will be XX or female. If the sperm cell contains a Y chromosome, then the resulting zygote will be XY or male.

Sex Chromosomes X-O: Grasshoppers, roaches, and other insects have a similar system for determining the sex of an individual. Adult males lack a Y sex chromosome and have only an X chromosome. They produce sperm cells that contain either an X chromosome or no sex chromosome, which is designated as O. The females are XX and produce egg cells that contain an X chromosome. If an X sperm cell fertilizes an egg, the resulting zygote will be XX or female. If a sperm cell containing no sex chromosome fertilizes an egg, the resulting zygote will be XO or male. Sex Chromosomes Z-W: Birds, insects like butterflies, frogs , snakes , and some species of fish have a different system for determining sex. In these animals it is the female gamete that determines the sex of an individual. Female gametes can either contain a Z chromosome or a W chromosome. Male gametes contain only the Z chromosome. Females of these species are ZW and males are ZZ. Parthenogenesis: What about animals like most kinds of wasps, bees, and ants that have no sex chromosomes? How is sex determined? In these species, fertilization determines sex. If an egg becomes fertilized, it will develop into a female.

A non-fertilized egg may develop into a male. The female is diploid and contains two sets of chromosomes , while the male is haploid . This development of an unfertilized egg into a male and a fertilized egg into a female is a type of parthenogenesis known as arrhenotokous parthenogenesis. Human Chromosomal Abnormalities: Sex Chromosome Abnormalities Sex Chromosome Abnormalities The majority of known types of chromosomal abnormalities involve sex chromosomes . In frequency of occurrence, they are only slightly less common than autosomal abnormalities. However, they are usually much less severe in their effects. The high frequency of people with sex chromosome aberrations is partly due to the fact that they are rarely lethal conditions. Like Down syndrome and other autosomal problems, sex chromosome gross abnormalities can be diagnosed before birth by amniocentesis and chorionic villi sampling. Sex chromosome abnormalities are gender specific. Normal males inherit an X and a Y chromosome while females have two X's. A single Y chromosome is sufficient to produce maleness while its absence is necessary for femaleness. Female abnormalities are due to variations in the number of X chromosomes.

Male abnormalities are the result of irregular numbers of either the X or the Y chromosome or both. Female Sex Chromosome Abnormalities Turner syndrome occurs when females inherit only one X chromosome--their genotype is X0 (i.e., monosomy X). If they survive to birth, these girls have abnormal growth patterns. They are short in stature, averaging 4 foot 7 inches as adults, and often have distinctive webbed necks (i.e., extra folds of skin), small jaws, and high arched palates. They generally lack prominent female secondary sexual characteristics. They have exceptionally small, widely spaced breasts, broad shield-shaped chests, and turned-out elbows. Their ovaries do not develop normally and they do not ovulate. The few o�cytes that they produce are destroyed by the time they are two. They are in a sense postmenopausal from early childhood and are sterile. However, they can become pregnant and give birth if fertilized eggs from a donor are implanted. Women with Turner syndrome have a higher than average incidence of thyroid disease, vision and hearing problems, heart defects, diabetes , and other autoimmune disorders . In a few individuals, there is slight mental retardation. Turner syndrome is rare.

Current estimates of its frequency range from 1 in 2,000 to 1 in 5,000 female infants. If diagnosed in early childhood, regular injections of human growth hormones can increase their stature by a few inches. Beginning around the normal age of puberty, estrogen replacement therapy can result in some breast development and menstruation. These treatments allow Turner syndrome women to appear relatively normal. Turner Syndrome --characteristics of Turner Syndrome and treatment for it. This link takes you to an external website. To return here, you must click the "back" button on your browser program. (length = 2 mins, 39 secs) Triple-X syndrome occurs in women who inherit three X chromosomes--their genotype is XXX or more rarely XXXX or XXXXX. As adults, these "super-females" or "metafemales" , as they are sometimes known, generally are an inch or so taller than average with unusually long legs and slender torsos but otherwise appear normal. They usually have normal development of sexual characteristics and are fertile but tend to have some ovary abnormalities that can lead to premature ovarian failure.

They may have slight learning difficulties, especially in speech and language skills, and are usually in the low range of normal intelligence (especially the XXXX and XXXXX individuals). They frequently are very tall in childhood and tend to be emotionally immature for their size. This sometimes results in teachers and other adults labeling them as troublemakers because they expect more maturity from bigger girls. However, they are usually as emotionally mature as other girls of their age. None of these traits prevent them from being socially accepted as ordinary adult women. Individuals who are genetic mosaics (XX/XXX) have less noticeable symptoms. Triple-X syndrome is less rare than Turner syndrome, but little is known about it. The frequency is approximately 1 in 1,000 female infants and it occurs more commonly when the mother is older. Male Sex Chromosome Abnormalities Klinefelter syndrome males inherit one or more extra X chromosomes--their genotype is XXY or more rarely XXXY or XY/XXY mosaic. In severe cases, they have relatively high-pitched voices, asexual to feminine body contours as well as breast enlargement, and comparatively little facial and body hair. They are sterile or nearly so, and their testes and prostate gland are small.

As a result, they produce relatively small amounts of testosterone . The feminizing effects of this hormonal imbalance can be significantly diminished if Klinefelter syndrome boys are regularly given testosterone from the age of puberty on. Like triple-X females (described above), many Klinefelter syndrome men are an inch or so above average height. They also are likely to be overweight. They usually have learning difficulties as children, especially with language and short-term memory. If not given extra help in early childhood, this often leads to poor school grades and a subsequent low self esteem. However, most men who have Klinefelter syndrome are sufficiently ordinary in appearance and mental ability to live in society without notice. It is not unusual for Klinefelter syndrome adults with slight symptoms to be unaware that they have it until they are tested for infertility. They are usually capable of normal sexual function, including erection and ejaculation, but many, if not most, are unable to produce sufficient amounts of sperm for conception. Klinefelter syndrome males with more than two X chromosomes usually have extreme symptoms and are often slightly retarded mentally. Men who are mosaic (XY/XXY) generally have the least problems.

There is no evidence that Klinefelter syndrome boys and men are more inclined to be homosexual, but they are more likely to be less interested in sex. They have a higher than average risk of developing osteoporosis , diabetes, and other autoimmune disorders that are more common in women. This may be connected to low testosterone production. Subsequently, regular testosterone therapy is often prescribed. The frequency of Klinefelter syndrome has been reported to be between 1 in 500 and 1 in 1000 male births. This makes it one of the most common chromosomal abnormalities. Males with Down syndrome sometimes also have Klinefelter syndrome. Both syndromes are more likely to occur in babies of older mothers. Klinefelter Syndrome --traits described by a doctor of medical genetics This link takes you to an external website. To return here, you must click the "back" button on your browser program. (length = 1 mins, 48 secs) XYY syndrome males inherit an extra Y chromosome--their genotype is XYY. As adults, these "super-males" are usually tall (above 6 feet) and generally appear and act normal. However, they produce high levels of testosterone.

During adolescence, they often are slender, have severe facial acne, and are poorly coordinated. They are usually fertile and lead ordinary lives as adults. Many, if not most, are unaware that they have a chromosomal abnormality. The frequency of XYY syndrome is not certain due to statistical differences between different studies. It may be as common as 1 in 900 male births to as rare as 1 in 1500 or even 1 in 2,000. XYY syndrome is also referred to as Jacobs syndrome. Early studies of XYY syndrome done in European prisons initially led to the erroneous conclusion that these men were genetically predisposed to antisocial, aggressive behavior, below average intelligence, and homosexuality. Contributing to the early view that XYY syndrome men have serious personality disorders was the case of Richard Speck. In 1966, he coldly murdered 8 nurses in a Chicago dormitory. At his trial, his lawyer claimed that he was innocent due to uncontrollable urges caused by his XYY genotype. This novel appeal was akin to claiming insanity or severely diminished mental competence. The jury was not convinced and found him guilty of murder. He was sentenced to life in prison where he eventually died.

In fact, Richard Speck did not have an XYY genotype. However, some researchers suggest that the high testosterone levels of XYY men can make them somewhat more prone to violence and that this may cause higher rates of wife beating. WHO | Gender and Genetics Gender and Genetics Genetic Components of Sex and Gender Humans are born with 46 chromosomes in 23 pairs. The X and Y chromosomes determine a person’s sex. Most women are 46XX and most men are 46XY. Research suggests, however, that in a few births per thousand some individuals will be born with a single sex chromosome (45X or 45Y) (sex monosomies) and some with three or more sex chromosomes (47XXX, 47XYY or 47XXY, etc.) (sex polysomies). In addition, some males are born 46XX due to the translocation of a tiny section of the sex determining region of the Y chromosome. Similarly some females are also born 46XY due to mutations in the Y chromosome.

Clearly, there are not only females who are XX and males who are XY, but rather, there is a range of chromosome complements, hormone balances, and phenotypic variations that determine sex. The biological differences between men and women result from two processes: sex determination and differentiation.(3) The biological process of sex determination controls whether the male or female sexual differentiation pathway will be followed. The process of biological sex differentiation (development of a given sex) involves many genetically regulated, hierarchical developmental steps. More than 95% of the Y chromosome is male-specific (4) and a single copy of the Y chromosome is able to induce testicular differentiation of the embryonic gonad. The Y chromosome acts as a dominant inducer of male phenotype and individuals having four X chromosomes and one Y chromosome (49XXXXY) are phenotypically male. (5) When a Y chromosome is present, early embryonic testes develop around the 10th week of pregnancy. In the absence of both a Y chromosome and the influence of a testis-determining factor (TDF), ovaries develop. Gender, typically described in terms of masculinity and femininity, is a social construction that varies across different cultures and over time.

(6) There are a number of cultures, for example, in which greater gender diversity exists and sex and gender are not always neatly divided along binary lines such as male and female or homosexual and heterosexual. The Berdache in North America, the fa’afafine (Samoan for “the way of a woman”) in the Pacific, and the kathoey in Thailand are all examples of different gender categories that differ from the traditional Western division of people into males and females. Further, among certain North American native communities, gender is seen more in terms of a continuum than categories, with special acknowledgement of “two-spirited” people who encompass both masculine and feminine qualities and characteristics. It is apparent, then, that different cultures have taken different approaches to creating gender distinctions, with more or less recognition of fluidity and complexity of gender. XYY Males Case Example 1: Genetics as an important determinant of biological sex In 1959, chromosomal analysis of two human disorders, Turner syndrome and Klinefelter syndrome, demonstrated for the first time that genetic factors on the Y chromosomes of mammals are important determinants in male sex.

(7) The sex determining region on the Y chromosome, referred to as the SRY, has a fundamental role in sex determination and is believed to be the switch that initiates testis development. It is, therefore, responsible for the initiation of male sex determination during embryo development. (8) SRY-box-related (SOX) genes have been identified on autosomes (9) and mutations in the SRY or SOX gene have been implicated in sex reversal. (10) In other words, individuals with SRY mutations should develop as females. (11) However, it is likely that there are other sex determining genes involved in phenotypic sex. (12) The DNA sequence of the human X chromosome and male-specific region of the Y chromosome have recently been published, enabling further research into sex-specific genes. (13) Genetically Determined Disorders of Sexual Differentiation Typical sexual development is the result of numerous genes, and mutation in any of these genes can result in partial or complete failure of sex differentiation. These include mutations or structural anomalies of the SRY region on the Y chromosome resulting in XY gonadal dysgenesis, XX males, or XY females; defects of androgen biosynthesis or androgen receptors, and others.

Androgen Insensitivity Syndrome ELSI of Genetic Components of Sex and Gender The issues of gender assignment, gender verification testing, and legal definitions of gender are especially pertinent to a discussion on the ELSI of gender and genetics. These practices, however, are misnomers as they actually refer to biological sex and not gender. Such a discrepancy is highlighted by the existence of intersex individuals whose psychosexual development and gender sometimes do not match the biological sex assigned to them as infants. In this report the term ‘sex’ will be used where the practice refers to biological sex and not the more social construct of ‘gender’. Sex Chromosome Abnormalities Chromosomes are the structures that carry genes which in turn transmit hereditary characteristics from parents to offspring. Humans have 23 pairs of chromosomes, one half of each pair inherited from each parent. The Y chromosome is small, carries few genes, and has abundant repetitive sequence, while the X chromosome is more autosome-like in form and content. (14)Despite being relatively gene-poor overall due to reduced recombination, the X and Y sex chromosomes are enriched for genes that relate to sexual development.

(15) Aneuploidy is the condition of having less than (monosomy) or more than (polysomy) the normal diploid number of chromosomes. Aneuploidy occurs in at least 5% of all pregnancies and is the most commonly recognized chromosome abnormality in humans. (16) Divergence from the normal number of X and Y chromosomes, called sex chromosome aneuploidy (SCA), accounts for approximately half of all chromosomal anomalies in humans with a total frequency of 1:400. (17) Sex chromosome physical abnormalities are able to be diagnosed prenatally via amniocentesis and chorionic villi sampling (CVS). Prenatal diagnosis of SCA is increasing because of the widespread use of these technologies. (18) The high frequency of individuals with SCA is due to the fact that their effects are generally not as severe as autosomal abnormalities and are rarely lethal. Indeed, most cases of SCA are compatible with normal life expectancy and often go undiagnosed. (19) Still, it is estimated that1 in 3 miscarriages is due to aneuploidy affecting the foetus.

(20) The induced abortion rate for SCA has steadily decreased from 100% in the 1970s to 69% in the 1980s and 49% in the 1990s. (21) It has been suggested that this decrease is directly the result of improved knowledge of the conditions associated with SCA combined with the influence of genetic counseling. (22) Turner syndrome This disorder, also referred to as monosomy X (45X) occurs in individuals that have one X chromosome, no Y chromosome, and are phenotypically female. Although 45X is a frequent chromosomal anomaly, Turner syndrome is rare with a live-birth frequency of 1:3000, (23) as only 1 in 40 affected zygotes develops to term. (24) Affected individuals experience abnormal growth patterns, are short in stature, generally lack prominent female secondary sexual characteristics and are sterile. In some instances of Turner syndrome, there is slight mental retardation. XXX Females Women with three X chromosomes (47XXX) experience normal development of sexual traits and are fertile.

Affected individuals are usually taller than average and have slender builds. The frequency of women obtaining an extra X chromosome is approximately 1:1000. There is no severe phenotype associated with three X chromosomes in women. (25) These women may have slight learning difficulties. Klinefelter Syndrome Klinefelter syndrome (47XXY or XY/XXY mosaic) with male phenotype is the most pervasive sex chromosomal anomaly (26) affecting approximately 1:600 males. (27) Males with Klinefelter syndrome carry two or more X chromosomes which results in abnormal development of the testis, leading to hypogonadism and infertility. (28) Affected individuals are often tall and produce relatively small amounts of testosterone. As a result of this hormone imbalance, affected males have incompletely developed secondary male sex characteristics. XYY Males Men inheriting an additional Y chromosome are usually taller than average and are prone to acne because they produce higher than average levels of testosterone. Affected males are typically fertile and many are unaware that they have a chromosomal abnormality. The frequency of males born with an additional Y chromosome is approximately 1:1000.

Hermaphroditism ‘True’ hermaphroditism is a genetic condition in which affected individuals have both mature ovarian and testicular tissue. (29) There are no published population-wide estimates of the frequency of true hermaphrodites. (30) The autosomal inheritance of this condition suggests that genes controlling sexual development and differentiation are not limited to the sex chromosomes.(31) Blackless et al. suggest that such familial inheritance “opens the possibility that, as with other inherited forms of sexual ambiguity, there may be pockets, perhaps even large geographical regions, with relatively high frequencies of true hermaphroditism.” (32) Congenital Adrenal Hyperplasia Congenital adrenal hyperplasia (CAH) is an inherited autosomal recessive condition that can affect both boys and girls. It is the most common cause of intersexuality in females with 46XX, where untreated girls develop an outwardly male appearance. This disorder, also called adrenogenital syndrome (AGS), results from a genetically caused deficiency of cortisol, a steroid hormone produced by the adrenal cortex.

The disorder occurs with a frequency of 1:5000 and results in incomplete female sex differentiation and increased androgenic effects due to a compensatory increase in adrenocortical hormone (ACTH). (33) Androgen Insensitivity Syndrome Androgen Insensitivity Syndrome (AIS) is an X-linked recessive disorder in which affected individuals have external female genitalia and breast development despite being genetically male (46XY). Tissues of affected individuals are unresponsive to male hormones (androgens) yet respond to estrogens. Wisniewski et al. assessed the physical and psychosexual status of 14 women with complete androgen insensitivity syndrome (CAIS) by questionnaire as well as medical examination in order to determine the participants' knowledge of their disorder as well as their opinion of medical and surgical treatment. (34) Most of the women questioned expressed satisfaction with their psychosexual development and sexual function. All of the women who participated in the study were satisfied at having been raised as females, and none of the participants desired gender reassignment. (35) Gender Assignment of Intersex Infants and Children Intersex is defined as a congenital anomaly of the reproductive and sexual system.

An estimate about the birth prevalence of intersex is difficult to make because there are no concrete parameters to the definition of intersex. The Intersex Initiative, a North-American based organization, estimates that one in 2,000 children, or five children per day in the United States, are born visibly intersex. (36) This estimate sits within range; from genital anomalies, such as hypospadias, with a birth prevalence of around 1:300 to complex genital anomalies in which sex assignment is difficult, with a birth prevalence of about 1:4500. (37) Many intersex children have undergone medical intervention for health reasons as well as for sociological and ideological reasons. An important consideration with respect to sex assignment is the ethics of surgically altering the genitalia of intersex children to “normalize” them. Clitoral surgery for intersex conditions was promoted by Hugh Hampton Young in the United States in the late 1930s. Subsequently, a standardized intersex management strategy was developed by psychologists at Johns Hopkins University (USA) based on the idea that infants are gender neutral at birth. (38) Minto et al.

note that “the theory of psychosexual neutrality at birth has now been replaced by a model of complex interaction between prenatal and postnatal factors that lead to the development of gender and, later, sexual identity”. (39) However, currently in the United States and many Western European countries, the most likely clinical recommendation to the parents of intersex infants is to raise them as females, often involving surgery to feminize the appearance of the genitalia. (40) Minto et al. conducted a study aiming to assess the effects of feminizing intersex surgery on adult sexual function in individuals with ambiguous genitalia. As part of this study, they noted a number of ethical issues in relation to this surgery, including that: there is no evidence that feminizing genital surgery leads to improved psychosocial outcomes; feminizing genital surgery cannot guarantee that adult gender identity will develop as female; and that adult sexual function might be altered by removal of clitoral or phallic tissue. (41) Developmental biology suggests that a strict belief in absolute sexual dimorphism is incorrect. (42) Instead, Blackless et al. suggest two overlapping bell-shaped curves to conceptualize sexual variations across populations.

Qualitative variation in chromosome complement, genital morphology and hormonal activity falls under the area of overlap. (43) Such an opinion challenges the need for medical intervention in cases of intersexuality. Legal Definitions of Gender An example of the implications of legal definitions of sexual affiliation In addition to social implications, sex and gender categorization has important political and legal implications as well. The identification of an individual as either biologically male or female can have legal ramifications for marriage licenses, spousal support and eligibility for parenthood. The issue of sexual classification, however, is complicated by factors such as chromosomal complement, external genitalia, gender identification and surgical alteration. In the United States, as in most countries, there is an absence of controlling legislation that defines exactly how a person’s gender is determined. (44) The following example demonstrates some of the complexities this may raise. In 1999, the Texas Court of Appeals considered the validity of a marriage between a man and a person born genetically as a man, but surgically altered to have the physical characteristics of a woman.

(45) As in most American states, Texas family law prohibits same-sex marriages, and the United States Congress has passed the Defense of Marriage Act that, for federal purposes, defines marriage as a “legal union between one man and one woman”. (46) Despite the fact that medical experts gave testimony to the effect that the individual in question was psychologically and psychiatrically female before and after the sex reassignment surgery, the court held that chromosome complement, and not sex reassignment surgery, outward gender characteristics, nor psychosocial self-identification, determine a person’s gender. The court invalidated the marriage on the grounds that the transsexual woman was legally a man. (47) This case is one example of how the legal system in one country has dealt with the complexities of defining sex and gender and the implications of defining gender and sex. Clearly, there are many other potential legal implications of such definitions and the complicated issues they raise are many.

Chromosomes X and Y and Sex Determination - Biology Online Chromosomes X and Y and Sex Determination Chromosomes X and Y and Sex Determination - Genetics and Evolution In a human, the normal chromosomes complement is 46, 44 of which are autosomes while 2 distinct chromosomes are deemed sex chromosomes , which determine the sex of an organism and various sex linked characteristics. In most animals, those who possess XX chromosomes are female while male animals possess an X and a Y chromosome. However, this is not true of all organisms, as it can be reversed in some species. Sex Determination A humans' sex is predetermined in the sperm gamete . The egg gamete mother cell is said to be homogametic , because all its cell possess the XX sex chromosomes. sperm gametes are deemed heterogametic because around half of them contain the X chromosome and others possess the Y chromosome to compliment the first X chromosome. In light of this, there are two possibilities that can occur during fertilisation between male and female gametes, XX and XY. Since sperm are the variable factor (i.e. which sperm fertilises the egg) they are responsible for determining sex.

Chromosomes X and Y Chromosomes X and Y do not truly make up a homologous pair. They act similarly in their roles, but they are not homologous (the same). The X chromosome in humans is much longer than the Y chromosome and also contains many more genes. These genes are said to be sex linked, due to the fact they are present in one of the sex chromosomes. During fertilisation, when the opposing homologous chromosomes come together, the smaller Y chromosome offers no dominance against the 'extra' X chromosomes as indicated below. The arrows indicate sex linked genes in the X chromosome. In this homologous pairing, all those genes are dominant, because there are no opposing genes in the Y chromosome to offer dominance. So when the organism has an XY chromosome compliment (i.e. a male), these sex linked genes are freely expressed in the organisms phenotype , an example being hairy ears developing in old age. Sex Linked Characteristics These sex linked genes on the X chromosome display a number of characteristics.

The following are just some examples of phenotypes as a result of these genes in expression; Red-Green colour blindness In humans gender is determined by the sex chromosomes the X and - IB - 100 View Full Document In humans, gender is determined by the sex chromosomes, the X and Y chromosomes. The X and Y chromosomes are very different in size and number of gene locations.  The X chromosome is much larger than the Y and carries about 1,400 gene locations.  The Y chromosome carries only about 200 gene locations. Genes on the Y chromosome determine whether an embryo develops as a male or female. All human embryos begin development with the beginnings of female structures. Embryos with two X chromosomes develop as females. The sex determining region of the Y chromosome, when present in an embryo, activates the formation of male structures. Since a female has two X chromosomes, she can only produce eggs, each with an X chromosome. Males have an X and a Y chromosome, so sperm may include either an X or a Y chromosome. An egg has a 50% chance being fertilized by an X carrying sperm, and the same 50% chance for a Y carrying sperm.

Some genes located on the X chromosome are inherited differently than genes located on one of the 22 autosomes because the Y chromosome is missing 1,200 genes that are on the X chromosome. Hemophilia A is caused by the inability to produce an important protein needed for the blood to clot properly. The gene for this protein is located on the X chromosome. There is no gene locus for the hemophilia A protein on This preview has intentionally blurred sections. Sign up to view the full version. View Full Document the Y chromosome. Thus, males are more likely to be affected by Hemophilia A than females. A female has two copies of the X chromosome so she also has two alleles of any gene located on the X chromosome. This means, that for a recessively inherited condition controlled by an X-linked gene, like Hemophilia A, a female may be:  Homozygous dominant : healthy  Heterozygous : healthy, but a carrier who can pass on the recessive allele. A female who is a carrier can produce eggs that carry the dominant allele or the recessive allele with a 50/50 chance.

 Homozygous recessive : affected  A male has a Y chromosome, which has no gene locus for hemophilia A. If a male embryo is formed from an egg carrying a dominant allele for hemophilia A, the boy will be healthy. If a male embryo is formed from an egg carrying a recessive allele for hemophilia A, the boy will develop the disorder. There is no allele on the Y chromosome to counteract a recessive allele on the X chromosome. Click on the image below to view a Biology Now animation of X-linked recessive disorder inheritance (3:15) Genes located close to each other on the same chromosome are usually inherited together. Typically genes are independently sorted into gametes, but there is an exception to the Law of Independent Assortment. Genes located near each other on a chromosome do not sort independently because when crossing over occurs, those genes tend to get swapped together in the section of DNA that exchanges between homologous chromosomes.

(See the figure and the animation below for a refresher about crossing over.) Thus, genes located close to each other on the same chromosome are usually inherited together, and closely linked alleles (variations of the same gene) tend to be inherited together more often than alleles that are farther apart. This explains why some This is the end of the preview. Sign up to access the rest of the document. TERM Name of your Genetic Disorder.docx UIllinois 1 Name of your Genetic Disorder Sandhoff Disease (similar to Tay- Sachs disease) 2 G Name of your Genetic Disorder.docx Evolution of the Y Chromosome | HHMI BioInteractive Evolution of the Y Chromosome This is part of: The Meaning of Sex: Genes and Gender Summary How did the human Y chromosome become so small relative to its X counterpart? This animation depicts the 300-million-year odyssey of the sex chromosomes that began when the proto X and Y were an identical pair. Animation: 125 other people found this useful Share This More About Evolution of The Y Chromosome Background The human X and Y chromosomes are a unique pair. The other chromosome pairs, called the autosomes , appear to be identical twins; they are superficially indistinguishable.

In contrast, the X and Y chromosomes appear to be vastly different from one another. Why are the sex chromosomes so different? How did they get that way? The Y chromosome is only one-third the size of the X. Although the Y has a partner in X, only the tips of these chromosomes are able to recombine. Thus, most of the Y chromosome is inherited from father to son in a pattern resembling asexual, not sexual, reproduction. No recombination means no reassortment, so deleterious mutations have no opportunity to be independently selected against. The Y chromosome therefore tends to accumulate changes and deletions faster than the X. Degradation doesn't occur in X chromosomes because during female meiosis , the X has the other X as a full partner in recombination. Clues of how the Y chromosome evolved can be found by comparing the genes and the sequences of X and Y chromosomes as well as homologous genes of different species. One method scientists use to estimate evolutionary time is observing how homologous genes have become different over time in different species. All DNA sequences accumulate random mutations over time, so species that are distant relatives should have more different sequences than close relatives because they have been evolving separately for a longer time.

Once recombination stopped between portions of X and Y, genes located on those parts started to evolve separately as homologs. Apparently, this happened in stages, so some X-Y gene pairs are more related than others, meaning they stopped recombining more recently. Also, chunks of genes stopped recombining, and by mapping their positions on the chromosome, one can guess that an event, like an inversion , may have taken place. Travel back in time, when human ancestors were reptile-like forms, and peer into the processes that shaped the X and Y chromosomes. Introduction How did the human Y chromosome become so small relative to its X counterpart? This animation depicts the 300-million-year odyssey of the sex chromosomes that began when the proto X and Y were an identical pair. Over time, structural changes in the Y chromosome resulted in its current form, which is specialized to trigger male development. The evolutionary timescale is represented by positioning the chromosomal remodeling events along an abbreviated vertebrate cladogram, a chart of evolutionary relationships. (Ma = million years ago) Part 1. Sex chromosomes originated as autosomes The sex chromosomes began as an ordinary pair of autosomes . During meiosis .

chromosomes replicate their DNA, pair, and exchange genes ( recombination ; red lines). A mutation in the SOX3 gene produced the SRY gene, a critical determinant of maleness, on the proto Y. While the functions of SRY and SOX3 became very different over time, another gene, RPS4 , retained a similar function on both the X and Y chromosomes. Part 2. Inversions restrict recombination between the X and the Y chromosome Inversions , which are internal recombination events, caused a rearrangement of genes on the Y chromosome. These rearrangements meant that large portions of the X and Y chromosome no longer recombined, which made the Y chromosome susceptible to deletions , and it decreased in size. Part 3. Comparison of sex-chromosome recombination in males and females After our lineage diverged from the ancestors of the monotremes, such as the duck-billed platypus, another inversion further scrambled the genes on the proto Y. In males, only the tips of the Y chromosome were left able to recombine with homologous genes on the X chromosome. In contrast, in females, recombination continued to occur across the full length of the two identical X chromosomes. Part 4.

Autosomal expansion of X and Y chromosomes About 130 million years ago (Ma), an autosome donated a block of genes that extended the length of both the X and the Y chromosome. The X and Y were able to recombine in these expanded regions of the chromosomes. Subsequently, inversions rearranged the order of genes on the Y chromosome. Additional rearrangements occured almost exclusively on the Y. Without recombination to preserve its integrity, the Y continued to lose genes and, over time, shrank. Part 5. An autosome contributed a copy of the DAZ spermatogenesis gene to the Y chromosome Sometime after squirrel monkeys diverged from the primates that evolved into humans, an autosome contributed a copy of the DAZ spermatogenesis gene to the Y chromosome. The DAZ gene was copied and copied again and now the modern Y chromosome contains four identical DAZ gene sequences. The modern Y chromosome is about one-third the size of its X-chromosome partner. Learn More: Autosomes An autosome is any chromosome that is not a sex chromosome. In ancient reptilian creatures, there was no chromosomal basis for sex determination. Scientists speculate that sex was determined by environmental factors such as temperature.

Some modern reptiles, including turtles and crocodiles, still use this mode of sex determination. Learn More: DAZ Many genes essential for the production of sperm are located exclusively on the Y chromosome. One of these genes, DAZ (deleted in azoospermia), was copied from an autosome and was copied twice on the Y chromosome, resulting in four copies of the DAZ gene. The Y chromosome is unique because not only are male spermatogenesis genes sequestered on the Y, but they exist as mutiple copies. In fact, the abundance of multiple copies and mirror images of sequences have led researchers to call the Y chromosome a "hall of mirrors." Although this sequence repetition created great challenges in the sequencing of the Y chromosome, the complex structure also serves an important purpose. Multiple copies of essential spermatogenesis genes ensure that in spite of deletion events, which may result in the loss of a single copy of an essential gene, spermatogenesis can still proceed via proteins produced by remaining copies. Learn More: Deletions Deletions are uncommon, but relative to inversions, they are not rare events. (Recombination, however, is a common event).

Deletions occur particularly in regions of the Y chromosome that do not undergo recombination. The chromosome is mutated, causing a section of DNA to be excised, and the two flanking ends of DNA join to form a continuous strand. Learn More: Expansion About 130 Ma, an autosome donated a block of genes that extended the length of both proto X and Y (expansion). The proto X and Y were able to recombine in these expanded regions of the chromosomes. Subsequently, inversions further rearranged the order of genes. Without recombination that preserved the integrity of chromosomes, the proto Y lost genes and, over time, shrank in size. Learn More: Inversions On an evolutionary timescale, large inversions, such as those shown in the animation, are actually very rare events. To occur, the DNA at one end of the chromosome recombines with DNA at the other end of the chromosome, forming a loop. Instead of the loop being eliminated from the chromosome (as happens in other types of recombination events), the loop twists. Therefore, the same DNA sequences are retained in the chromosome, but their orientation is reversed.

Scientists speculate that large inversions resulted in the relocation of SRY to the "top" of the Y chromosome relative to its former partner SOX3, which remains near the "bottom" of the X chromosome. Learn More: Meiosis To produce sperm or eggs, germ cells undergo the process called meiosis. Chromosomes replicate and pair up, resulting in a 4n quantity of chromosomes. Two nuclear divisions follow, so that the gametes have a haploid (1n) number of chromosomes. Learn More: Mutation Mutations are alterations in the DNA sequence that occur randomly and can have little or great consequences, depending on the location of the mutation. When a mutation altered SOX3, the testes-determining gene SRY resulted. This mutation had great consequences: The evolution of unique sex chromosomes began. Learn More: Recombination During meiosis, chromosomes with substantial DNA sequence homology will pair and exchange pieces of DNA, a process called genetic recombination. This process provides not only a source of genetic variability but also a way in which deleterious mutations are eliminated by not being passed on to future generations.

(Recombination allows for reassortment and thus a way for selecting out deleterious mutations without needing to eliminate the entire set of alleles on which the mutation arose.) In this way, recombination preserves the integrity of the chromosomes. The X and Y chromosomes are notably different with respect to recombination. During female meiosis, the two X chromosomes undergo recombination throughout their entire length (illustrated by red lines). In contrast, during male meiosis, the Y chromosome recombines with the X chromosome only at its tips. Thus, over time, deleterious mutations accumulate in the nonrecombining regions of the Y chromosome. Learn More: RPS4 RPS4 (ribosomal protein small subunit, protein 4) is a gene essential for ribosome formation. Identical copies of RPS4 are found on both the X and the Y chromosome, and the same function has been retained over timeósince before the creation of SRY and through millions of years of divergence between the X and Y. However, the location of RPS4 has changed as a result of a large inversion. Learn More: SRY The SRY gene regulates the formation of the testes from the undifferentiated embryonic gonad.

The location of SRY and SOX3 on the X and Y chromosomes of several species has been compared, an analysis that has allowed scientists to roughly date when changes occurred. Gene pairs, such as SRY and SOX3, allow researchers to map changes and rearrangements on chromosomes. Specifically, 300 million years ago (Ma), scientists speculate that SOX3 was in the same location on the proto X and Y chromosomes. A mutation in SOX3 created the gene SRY on the Y chromosome. Researchers found that monotremes are the most ancient mammals that have the SRY gene, whereas all earlier ancestors do not. These data allowed researchers to estimate when the mutation that created SRY occurred. References Used in Developing the Animation Jegalian, K., and Lahn, B.T. 2001. Why the Y is so weird. Scientific American 284(2): 56–61.

Kuroda-Kawaguchi, T., Skaletsky, H., Brown, L.G., Minx, P.J., Cordum, H.S., Waterston, R.H., Wilson, R.K., Silber, S., Oates, R., Rozen, S., and Page D.C. 2001. The AZFc region of the Y chromosome features massive palindromes and uniform recurrent deletions in infertile men. Nature Genetics 29(3): 279–86. Lahn, B.T., and Page, D.C. 1999. Four evolutionary strata on the human X chromosome. Science 286(5441): 964–7. Lahn, B.T., Pearson, N.M., and Jegalian, K. 2001. The human Y chromosome, in the light of evolution. Nature Reviews Genetics 2(3): 207–16. Credits Scientific Direction: David Page, M.D. Scientific Content: Donna Messersmith, Ph.D.

Animators: Chris Vargas, Eric Keller Downloads Chromosomes and Gender Chromosomes and Gender ï¿½ Mitosis Chromosomes are long, stringy aggregates of genes that carry heredity information. They are composed of DNA and proteins and are located within the nucleus of our cells. Chromosomes determine everything from hair color and eye color to gender. Whether you are a male or female depends on the presence or absence of certain chromosomes. Human cells contain 23 pairs of chromosomes for a total of 46. There are 22 pairs of autosomes and one pair of sex chromosomes. The sex chromosomes are the X chromosome and the Y chromosome. These chromosomes determine gender. Karyotype of a normal male with 22 pairs of autosomes and one pair of sex chromosomes. Image Credit: U.S. Department of Energy Human Genome Program, In human sexual reproduction, two distinct gametes fuse to form a zygote. Gametes are reproductive cells produced by a type of cell division called meiosis . They contain only one set of chromosomes and are said to be haploid (one set of 22 autosomes and one sex chromosome).

The male gamete, called the spermatozoan, is relatively motile and usually has a flagellum . The female gamete, called the ovum, is nonmotile and relatively large in comparison to the male gamete. When the haploid male and female gametes unite in a process called fertilization, they form what is called a zygote. The zygote is diploid, meaning that it contains two sets of chromosomes (two sets of 22 autosomes and two sex chromosomes). Sex Chromosomes X-Y The male gametes or sperm cells in humans and other mammals are heterogametic and contain one of two types of sex chromosomes. They are either X or Y. The female gametes or eggs however, contain only the X sex chromosome and are homogametic . The sperm cell determines the sex of an individual in this case. If a sperm cell containing an X chromosome fertilizes an egg, the resulting zygote will be XX or female. If the sperm cell contains a Y chromosome, then the resulting zygote will be XY or male. Sex Chromosomes X-O Grasshoppers, roaches, and other insects have a similar system for determining the sex of an individual.

Adult males lack a Y sex chromosome and have only an X chromosome. They produce sperm cells that contain either an X chromosome or no sex chromosome, which is designated as O. The females are XX and produce egg cells that contain an X chromosome. If an X sperm cell fertilizes an egg, the resulting zygote will be XX or female. If a sperm cell containing no sex chromosome fertilizes an egg, the resulting zygote will be XO or male. Sex Chromosomes Z-W Birds, insects like butterflies, and some species of fish have a different system for determining gender. In these animals it is the female gamete that determines the sex of an individual. Female gametes can either contain a Z chromosome or a W chromosome. Male gametes contain only the Z chromosome. Females of these species are ZW and males are ZZ. Parthenogenesis What about animals like most kinds of wasps, bees, and ants that have no sex chromosomes? How is gender determined? In these species, fertilization determines gender. If an egg becomes fertilized it will develop into a female. A non-fertilized egg may develop into a male. The female is diploid and contains two sets of chromosomes, while the male is haploid.

This development of an unfertilized egg into an individual is called parthenogenesis . Y chromosome - Genetics Home Reference Y chromosome Description The Y chromosome is one of the two sex chromosomes in humans (the other is the X chromosome). The sex chromosomes form one of the 23 pairs of human chromosomes in each cell. The Y chromosome spans more than 59 million building blocks of DNA (base pairs) and represents almost 2 percent of the total DNA in cells. Each person normally has one pair of sex chromosomes in each cell. The Y chromosome is present in males, who have one X and one Y chromosome, while females have two X chromosomes. Identifying genes on each chromosome is an active area of genetic research. Because researchers use different approaches to predict the number of genes on each chromosome, the estimated number of genes varies. The Y chromosome likely contains 50 to 60 genes that provide instructions for making proteins. Because only males have the Y chromosome, the genes on this chromosome tend to be involved in male sex determination and development. Sex is determined by the SRY gene, which is responsible for the development of a fetus into a male. Other genes on the Y chromosome are important for male fertility.

Many genes are unique to the Y chromosome, but genes in areas known as pseudoautosomal regions are present on both sex chromosomes. As a result, men and women each have two functional copies of these genes. Many genes in the pseudoautosomal regions are essential for normal development. Related Information 46,XX testicular disorder of sex development In most individuals with 46,XX testicular disorder of sex development, the condition results from an abnormal exchange of genetic material between chromosomes (translocation). This exchange occurs as a random event during the formation of sperm cells in the affected person's father. The translocation affects the gene responsible for development of a fetus into a male (the SRY gene). The SRY gene, which is normally found on the Y chromosome, is misplaced in this disorder, almost always onto an X chromosome. A fetus with an X chromosome that carries the SRY gene will develop as a male despite not having a Y chromosome. 47,XYY syndrome Males with 47,XYY syndrome have one X chromosome and two Y chromosomes in each cell, for a total of 47 chromosomes.

It is unclear why an extra copy of the Y chromosome is associated with tall stature, learning problems, and other features in some boys and men. Some males with 47,XYY syndrome have an extra Y chromosome in only some of their cells. This phenomenon is called 46,XY/47,XYY mosaicism. 48,XXYY syndrome 48,XXYY syndrome is caused by the presence of an extra X chromosome and an extra Y chromosome in a male's cells. Extra genetic material from the X chromosome interferes with male sexual development, preventing the testes from functioning normally and reducing the levels of testosterone (a hormone that directs male sexual development) in adolescent and adult males. Extra copies of genes from the pseudoautosomal region of the extra X and Y chromosome contribute to the signs and symptoms of 48,XXYY syndrome; however, the specific genes have not been identified. Y chromosome infertility Y chromosome infertility is usually caused by deletions of genetic material in regions of the Y chromosome called azoospermia factor (AZF) A, B, or C. Genes in these regions are believed to provide instructions for making proteins involved in sperm cell development, although the specific functions of these proteins are unknown.

Deletions in the AZF regions may affect several genes. The missing genetic material likely prevents production of a number of proteins needed for normal sperm cell development, resulting in an inability to father children. Chromosome: MedlinePlus Medical Encyclopedia URL of this page: //medlineplus.gov/ency/article/002327.htm Chromosome Chromosomes are structures found in the center (nucleus) of cells that carry long pieces of DNA. DNA is the material that holds genes . It is the building block of the human body. Chromosomes also contain proteins that help DNA exist in the proper form. Information Chromosomes come in pairs. Normally, each cell in the human body has 23 pairs of chromosomes (46 total chromosomes). Half come from the mother; the other half come from the father. Two of the chromosomes (the X and the Y chromosome) determine if you are born a boy or a girl (your gender). They are called sex chromosomes: Females have 2 X chromosomes. Males have 1 X and 1 Y chromosome. The mother gives an X chromosome to the child. The father may contribute an X or a Y. The chromosome from the father determines if the baby is a girl or a boy.

The remaining chromosomes are called autosomal chromosomes. They are known as chromosome pairs 1 through 22. Images

Question: A human with X and Y sex chromosomes is usually which gender?
Answer: Male